The invention relates to novel Thermoplastic Olefin compositions comprising polypropylene, and ethylene-alpha olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. The ethylene-propylene copolymer compatabilizer imparts a greater degree of compatibility between the polypropylene and elastomer phases yielding improved physical properties.
Thermoplastic Olefin Compositions (TPOs) are defined as blends of polypropylene, olefinic elastomers and optionally fillers and other compounding ingredients. TPOs are multiphase polymer blends where the polypropylene forms a continuous matrix phase and the elastomer and fillers are the dispersed components. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO while the elastomer imparts flexibility and impact resistance.
Traditionally, ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the elastomeric component in TPOs. Recently, other ethylene-alpha olefin copolymers have been used, especially ethylene-butene and ethylene-octene copolymers.
The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. These parts are generally made using injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate&gt;35). These high melt flow rate (MFR) resins are difficult to toughen, resulting in products that have low impact strength.
One of the reasons impact modification of high MFR polypropylene resins is difficult is because of the large differences in the melt viscosities between the polypropylene resins employed and the elastomers typically used as impact modifiers. These differences lead to a poor dispersion of the elastomer in the polypropylene matrix, resulting in large dispersed elastomer particle sizes which, in turn, is detrimental to overall impact strength.
One proposed solution to the problem has been to decrease the molecular weight of the elastomer used, in order to reduce the viscosity of the elastomer. While this route produces better dispersion of the elastomer in the polypropylene matrix, the reduced molecular weight of the modifier adversely affects the impact strength of the TPO.
Another proposed solution has been to develop products which behave like a low viscosity plastic during the mixing process, yet function like an elastomer in the molded TPO. These type of polymers are generally referred to as plastomers. To date, however, these plastomer products have not yielded satisfactory impact performance when used with high melt flow polypropylene.
A third area that has been explored is the use of branched elastomers. U.S. Pat. No. 5,681,897 discloses the use of substantially linear ethylene-alpha olefin copolymers having a degree of long chain branching as impact modifiers for polypropylene as well as other thermoplastic resins. While the use of these elastomers appears to lead to an improvement in impact strength, there is still a need for impact strength and flexibility in TPOs made with high MFR polypropylene resins.